Thickeners are regularly added to sheet molding compounds (SMC) or bulk molding compounds (BMC) in order to facilitate handling and adhesion between low molecular weight polymeric resin and fiber components during mold filling. The low molecular weight polymeric resins are increased in viscosity through polymerization. Typical of these low molecular weight polymeric resins are unsaturated polyester and vinyl ester. In the case of an unsaturated polymeric resin, the addition of di- or poly-functional species are cross-linked to form a branched network of polymer chains.
One type of thickener includes alkali earth oxides and hydroxides. Of these thickeners, magnesium oxide is the most commonly used of such thickeners. Generally, it is believed that alkali earth oxides and hydroxides form ionic bonds with functional moieties of polymeric resins present. The functional moieties illustratively include terminal carboxylate groups of polyester resins.
Diisocyanates represent another type of thickener. Diisocyanates form covalent urethane bonds with hydroxyl groups associated with polymeric resin. The inclusion of a diamine, diol, polyamine or polyol separate from the polyester resin allows thickening to occur through formation of an interpenetrating network relative to the polyester resin. While diisocyanate thickeners tend to increase viscosity rapidly and retain the viscosity over time, the material flow associated with diisocyanate thickened material within a mold is generally less than desirable. Exemplary compositions are detailed in U.S. Pat. Nos. 4,067,845; 4,073,828; 4,232,133; 4,296,020; 4,622,384; 5,100,935; and 5,268,400. Alternatively, alkali earth oxide or hydroxide thickeners tend to add viscosity more slowly and are sensitive to humidity even after ionic bond formation has occurred. Another feature of the ionic bonds formed through alkali earth oxide or hydroxide thickeners is that the bonds tend to weaken at the temperatures associated with molding thereby reducing compound viscosity and increasing material flow.
The prior art attempts have met with limited success in improving molding compound properties such that initial viscosity increases rapidly, is maintained in the presence of humidity, and viscosity reduces under molding conditions. These attempts have included the concept of using both alkali earth oxide or hydroxide thickeners and diisocyanate thickeners in the same system. E. G. Melby and J. M. Castro, Chapter 3, Comprehensive Polymer Science, Pergamon Press (1989). Unfortunately, such systems have met with limited commercial acceptance and have failed to attain practical usability. As isocyanate reaction with an additive of a diamine, diol, polyamine or polyol to form an interpenetrating network relative to the curable polyester resin thickens independent of terminal carboxylate groups of the polyester resin, these different modes of thickening are accepted to build viscosity cumulatively. However, the ability to form molding compounds that have a viscosity build to the formulation of between 20,000 centipoise (initial) and 130,000 centipoise (15 minutes thereafter) and building to between about 30,000,000 to 40,000,000 centipoise at 24 hours and have a viscosity that 10 days later is no more than 60,000,000 centipoise has not been achieved.
Thus, there exists a need for a new molding compound thickener that has reduced moisture sensitivity and a more stable viscosity over a wide temperature range, as compared to conventional alkali earth oxide or alkali earth hydroxide thickeners. There further exists a need to provide viscosity build to the formulation of between 20,000 centipoise (initial) and 130,000 centipoise (15 minutes thereafter) and building to ideally between about 30,000,000 to 40,000,000 centipoise at 24 hours and have a viscosity that 10 days later is no more than 60,000,000 centipoise.